1. Field of the Invention
The present invention relates to the design of Ethernet passive optical networks. More specifically, the present invention relates to a method and an apparatus for controlling transmission to reduce interference caused by laser noise from different nodes in an Ethernet passive optical network.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies that are presently being developed, Ethernet passive optical networks (EPONs) are one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, because Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and, consequently, require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically performed between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which is typically an external network belonging to an Internet Service Provider (ISP) or a local exchange carrier. An ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services. ONUs are typically coupled to a one-by-N (1×N) passive optical coupler, where N is the number of ONUs, and the passive optical coupler is typically coupled to the OLT through a single optical link. (Note that one may cascade a number of optical splitters/couplers to accommodate more ONUs.) This configuration can significantly save the number of fibers and the amount of hardware required by EPONs.
Communications within an EPON can be divided into downstream traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT). In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, downstream data frames are broadcast by the OLT to all ONUs and are subsequently extracted by their destination ONUs. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler with the OLT.
Correspondingly, an EPON typically employs some arbitration mechanism to avoid data collision and to provide fair sharing of the upstream fiber-channel capacity. This is achieved by allocating a transmission timeslot to each ONU. An ONU typically buffers data it receives from a subscriber until the ONU's local time reaches the start time of its transmission timeslot. When its turn arrives, the ONU “bursts” all stored frames to the OLT at full channel speed.
One issue in designing an EPON is the interference with data transmission caused by laser noise from different ONUs. A laser transmitter within an ONU generates spontaneous emission noise, even in the absence of data transmission. Hence, if an ONU's laser remains on between transmission timeslots, its spontaneous emission noise may impair the signal quality of data transmitted by another ONU. This impairment worsens when the noise from multiple closely-located ONUs interferes with the data signal from a distant ONU.
One can avoid the laser-noise problem by turning off a laser when it is not transmitting data. This can be accomplished by using a laser-control signal, which is generated in the data link layer and received in the physical layer.
However, sending a control signal across several sublayers violates a basic layering principle, which is, a layer or a sublayer may only signal its immediate neighboring layers or sublayers. Note that in an ONU, the data link layer includes a medium access control (MAC) client sublayer, a multi-point MAC control sublayer, and a MAC sublayer. Furthermore, the physical layer includes a reconciliation sublayer, a Gigabit media independent interface (GMII), a physical coding sublayer (PCS), a physical medium attachment (PMA) sublayer, and a physical medium dependent (PMD) sublayer. In this environment, a laser-control signal is typically generated by the MAC control sublayer and is received by the PMD sublayer, thereby bypassing the sublayers between the MAC control sublayer and the PMD sublayer.
Hence, what is needed is a method and an apparatus for efficiently controlling laser transmission for an ONU without sending a control signal across multiple sublayers.